1. Field of the Invention
This invention relates to gas turbine engines and more specifically to the blades of a rotor assembly of such a machine.
2. Description of the Prior Art
The rotor assembly of a gas turbine machine includes a rotor disk having an axis of rotation and a plurality of outwardly extending blades each disposed about a spanwisely extending axis. Each rotor blade has a root and an outwardly extending airfoil attached to the root. The disk is adapted to receive the root of the rotor blade.
In modern engines many rotor assemblies typically have shrouds extending between the airfoils of adjacent blades. Shrouds which extend laterally between adjacent rotor blades at some point between the tip of the rotor blade and the root of the rotor blade are called part span shrouds. Shrouds which extend laterally between the tips of adjacent rotor blades are called tip shrouds. An example of a tip shrouded blade is shown in U.S. Pat. No. 4,076,455 issued to Stargardter entitled, "Rotor Blade System For A Gas Turbine Engine". The shrouds are cantilevered from the suction side surface of each airfoil and from the pressure side surface of each airfoil. The shrouds of adjacent blades abut along a contact face. In other applications, part span shrouds may be used in combination with tip shrouds or may be used singularly.
In either case, the shrouds are valuable for several reasons. The shrouds tie together the blades in a row to allow every blade in the row to accept its share of mechanical loads which develop, for example, when the blades are struck by foreign objects. The shrouds control tip amplitude during flight conditions and reduce the tendency for blade deflection about the spanwise axis and minimize vibration of the rotor blades. Damping of the blade takes place through rubbing of the contact faces of adjacent shrouds. However, additional rotational loads are created by the added mass of the shrouds located near the tip and near the middle of the blade as compared with rotor blades having no shrouds. These rotational loads induce stresses at the shroud airfoil interface and the root slot interface of the rotor blade and the disk. These stresses in shrouded blades require more massive designs than non-shrouded blades of equivalent cyclic fatigue life. Accordingly, scientists and engineers are working to provide shrouded rotor blades having reduced mass as compared with conventionally shrouded rotor blades such that the rotor blade has an improved cyclic fatigue life.